It is known practice, in the cosmetics or dermatological field, to use oil-in-water (O/W) emulsions. These emulsions, that have an oil phase (or lipophilic phase) dispersed in an aqueous phase, have an external aqueous phase and are therefore products that are more pleasant to use because of the feeling of freshness that they provide. However, they have the drawback of relatively lacking stability when the amount of oil present is too great. Now, for some applications, it is advantageous to have a large amount of oils since the oils provide comfort for the skin, nourish it, and can also remove makeup from it when these oils have makeup-removing properties.
Moreover, it is advantageous to have fine emulsions, i.e. emulsions where the oily phase is in the form of very small droplets, i.e. of droplets less than 4 μm in size, since these fine emulsions have a pleasant cosmetic feel and are generally more stable than coarse emulsions.
These emulsions can be prepared in particular by the phase inversion temperature technique (PIT emulsions), in which the average size of the globules constituting the oily phase is within given limits, namely between 0.1 and 4 μm (100 to 4000 nm). The principle of phase inversion temperature (or PIT) emulsification is, in theoretical terms, well known to those skilled in the art; it was described in 1968 by K. Shinoda (J. Chem. Soc. Jpn., 1968, 89, 435). It was shown that this emulsification technique makes it possible to obtain stable fine emulsions (K. Shinoda and H. Saito, J. Colloid Interface Sci., 1969, 30, 258). This technology was applied in cosmetics as early as 1972 by Mitsui et al. (“Application of the phase-inversion-temperature method to the emulsification of cosmetics”; T. Mitsui, Y. Machida and F. Harusawa, American. Cosmet. Perfum., 1972, 87,33).
The principle of this technique is as follows: an O/W emulsion (introduction of the aqueous phase into the oily phase) is prepared at a temperature that should be greater than the phase inversion temperature of the system, i.e. the temperature at which the equilibrium between the hydrophilic and lipophilic properties of the emulsifier(s) used is attained; at higher temperature, i.e. greater than the phase inversion temperature (>PIT), the emulsion is of water-in-oil type and, as it cools, this emulsion inverts at the phase inversion temperature so as to become an emulsion of oil-in-water type, having beforehand passed through a state of microemulsion. This process makes it possible to readily obtain emulsions with a diameter generally less than 4 μm. Emulsifying surfactants of the oil-in-water type conventionally used have an HLB (HLB=hydrophilic lipophilic balance) ranging from 8 to 18. These emulsifiers, due to their amphiphilic structure, are situated at the oil phase/aqueous phase interface, and thus stabilize the dispersed oil droplets.
However, it is difficult to produce fine O/W emulsions containing a large amount of oily phase, since such emulsions have a tendency to destabilize, this destabilization resulting in coalescence and separation of the aqueous and oily phases with release of the oil. In order-to improve the stability of these emulsions, the concentration of emulsifiers can be increased; however, a high concentration of emulsifiers can result in a rough, clingy or sticky feel, and in problems of innocuity with respect to the skin, the eyes and the scalp.
Document WO-A-01/89678 describes oil-rich emulsions containing 70% of oil, this oil being dicaprylyl ether. However, the stability of these emulsions is not sufficient, as shown in the comparative examples presented below, in particular comparative example 1.
There therefore remains a need for providing fine O/W emulsions containing a large amount of oils while at the same time being stable.